Resins of aromatic polyimides are characterized by excellent physical and chemical properties and particularly by a high resistance to heat and to oxidation. Accordingly, they can be used in many applications for manufacturing films, varnishes or coatings withstanding high temperatures. But the aromatic polyimides are generally infusible and not or not very soluble in organic solvents. For this reason, the known techniques require the intermediary step of using a soluble, non-cyclized polymer or a mixture of fusible reactants which, by suitable thermal treatment, may give a polymer material.
The most common synthesis comprises a first step of preparing a soluble polyamide-acid by reacting the dianhydride of a tetracarboxylic aromatic acid with an aromatic diamine in a polar aprotic solvent. These intermediary linear polymers may be macromolecular compounds of high molecular weight when the reactants are used in proportions close to the stoichiometry. But they may also be oligomers of low molecular weight when using an excess of one of the reactants, and then the end-chain functional groups are reacted with compounds having a latent reactivity.
These two types of precursor polymers of aromatic polyimides suffer however from the same disadvantages, resulting in particular from the insolubility of the imide form and from the instability of the amide-acid form. As a matter of fact, the polyamide-acids are very sensitive to heat and moistness, thus requiring to be kept at low temperature, preferably lower than 5.degree. C., in anhydrous atmosphere and preferably under inert gas. The polyamide-acid resins are converted to polyimides by thermal or chemical cyclodehydration which is performed at the time of use. The cyclization occurs for example when the wet film of polyamide-acid is progressively heated from room temperature up to 300.degree. C. or 350.degree. C. This heating produced by the dehydration reaction leading to imide rings. As a general rule, this thermal treatment must be progressive and lasts several hours, for example 5-15 hours, in order to avoid the formation of bubbles or of important defects in the mass of the material. All this process explains that the polyamide-acids are not easily adapted to the manufacture of thick elements such as composite materials or moulded articles wherein the diffusion of the solvent or of the reaction water is very difficult, if not impossible.
Resins of aromatic polyimides of high molecular weight do exist which are soluble in organic solvents since the polymers forming them have dissymmetrical structures or carry bulky lateral groups, but these resins are not fusible even when their vitrous transition temperature is lower than 300.degree. C. As a matter of fact, for manufacturing composite materials, the resin to be used as binder must be, at one stage, sufficiently fluid to wet, as perfectly as possible, the carrier consisting of fibers, fabrics or fillers. Also its viscosity, in molten state, must be adapted to the control of the different parameters of the compression molding.
According to one of the approaches proposed in U.S. Pat. No. 3,528,950, a prepolymer of lower molecular weight is prepared by reacting an aromatic diamine with a dianhydride of tetracarboxylic aromatic acid and a monoanhydride of unsaturated cycloaliphatic dicarboxylic acid. The reaction is conducted by heating the three reactants in a polar organic solvent with distillation of the reaction water. But this method provides two types of polymers having very different molecular weights and which are separately isolated. The polymers of higher molecular weight precipitate in the reaction medium and the others subsequently. After drying, the two products are admixed, as powder, and shaped by pressing at a temperature from 200.degree. C. to 350.degree. C., temperature at which the imide oligomers are cross-linked by polymerization of the ethylenic double bonds forming the chain ends. This method has the advantage of avoiding the problems of instability of the polyamide-acid solutions, but it requires an additional step for isolating the two types of polymers and readmix them. Moreover, these oligomers are not soluble in any proportion in the organic solvents.
Another technique, which seems particularly interesting, is proposed in U.S. Pat. No. 3,745,149, wherein a composition is prepared by admixing a diester or a tetraester of tetracarboxylic aromatic acid with an aromatic diamine and a monoester or diester of unsaturated dicarboxylic cycloaliphatic acid. These three reactants are dissolved in an organic solvent of relatively low boiling point, such for example as methanol and this monomer solution is used for impregnating fibers or fabrics which form the reinforcement of composite materials.
After evaporation of the most part of the solvent, imide oligomers are formed by polycondensation of the reactants at the time of use of the materials. This method has the advantage of using a system formed of fusible, soluble reactants and is compatible with the present technical means. For all these reasons, these resins, called PRM ("polymerization of reactive monomers") are increasingly used.
It must be observed, however, that, in spite of their indeniable advantages, the resins of this type suffer from disadvantages associated to their method of preparation. The most important disadvantages are the following:
The monomer solutions and the pre-impregnated materials prepared from these solutions are not stable and the characteristics of the products vary during time. It is hence necessary to keep them at a low temperature and preferably sheltered from moistness.
The monomer solutions and the pre-impregnated materials prepared from these solutions contain a substantial amount of free aromatic diamines which are known as highly carcinogenic. The use of these products requires protections at all levels to avoid that the users be exposed to these diamines.
The composite materials are directly manufactured from fabrics preimpregnated with the mixture of reactants by using a heating and pressure adjustment program which is very difficult to control. As a matter of fact, an object of this program is to perform the oligomerization accompanied with the release of a substantial amount of volatile products (water and alcohol), to complete the drying of the impregnated material and to produce a flow of the resin before polymerization of the ethylenic groups.